Switch-mode RF power amplifiers (notably Class-D or Class E amplifiers) may be used in RF transmitters (notably in CMOS (complementary metal oxide semiconductor) transmitters) for short-range wireless communications (such as Bluetooth or Zigbee). As an example, an all-digital CMOS class D powers amplifier (comprising logic gates, inverters, NANDs and/or NORS) may be used for a Bluetooth low-energy (BLE) radio transceiver. The theoretical efficiency of such a power amplifier is 100%, because the square waveform which is processed by the power amplifier causes non-overlapping of voltage and current signals that delivers 100% in-band energy.
However, in practice, along with the efficient power transmission within the allocated in-band bandwidth (e.g. f0=2.400 GHz-2.484 GHz), unwanted spurious emissions in the out-of-band (OOB) domain typically needs to be controlled and/or attenuated to comply with regulatory emission limits. By way of example, the FCC (Federal Communications Commission) regulations restrict the BLE out-of-band harmonic (2nd, 3rd, 4th and other higher order harmonics) power level to be below a certain level (e.g. <−45 dBm), in order to avoid interferences with the operating frequencies of other cellular, territorial and/or wireless personal-area-networks.
However, switch-mode class D power amplifiers (PAs), which are operated with non-sinusoidal RF signals (e.g. rectangular or square signals), typically generate a relatively high level of OOB harmonics (e.g. at 2f0, 3f0, 4f0, 5f0) along with the wanted signal (being the fundamental f0). Theoretically, for ideal rectangular RF signals (with sharp rise and fall times) having a 50% duty cycle (i.e. having an even symmetry), only odd harmonics (3f0, 5f0) are generated. In practice, for non-ideal RF signals (with relatively long rise and fall times and with imperfections due to device and inter-connect parasitics) the duty cycles deviate from 50% and the even symmetry does not hold. This causes the generation of both odd and even harmonics.